Aquaculture Research, 1996, 27, 531-538 Feed consumption, growth and protein utiiization of Colossoma macropomum (Cuvier) at different dietary fish meal/soya meal ratios M B van der Meer,'-^ E A Huisman^ & M C J Verdegem^ 'Programa UNA-LUW, Escuela de Ciencias Biologicas. Universidad Nacional, Heredia, Costa Rica, and ^Department of Fish Culture and Fisheries, Wageningen Agricultural University, Wageningen, The Netherlands Correspondence: M B van der Meer, Department of Fish Culture and Fisheries. Wageningen Agricultural University, P 0 Box 3 3 8, 6 700 AH Wageningen, The Netherlands Abstract The effect of a gradual substitution of dietary fish meal protein by soya meal protein on growth, feed uptake and protein utilization of 1 g Colossoma macro- pomum (Cuvier) was studied at two different dietary protein levels. Growth rates of fish fed ad libitum 20 and 45% protein diets fluctuated between 41 and 49, and between 60 and 68 gkg-"" day-', respectively. Fish incorporated between 31 and 47% of the dietary protein in their body. Increased amounts of soya meal in the diet led to decreased feed uptake, higher body protein levels, lower ash levels and increased NPU values. If the low ash contents in the fish fed 100% soya diets are not prejudicial for health and growth of the fish in the long term, soya meal must be considered a superior protein source for C. macropomum. The high growth and the efficient use of the dietary protein indicate the C. macropomum is able to utilize soya protein more efficiently than other fish species. Introduction Under laboratory conditions, juvenile Colossoma macropomum (Cuvier) can attain growth rates up to 52-55 g kg-"" day-' (Gtinther & Boza 1993; Van der Meer, Machiels&Verdegeml995),These high growth rates require the use of dietary protein levels above 40% (Van der Meer et al. 1995), Because protein is the most expensive macro-nutrient in artificial fish diets (Pillay 1990), and because feeds constitute usually the major part of the production costs for intensive culture, the use of cheaper dietary protein sources could play a key role in optimizing the econotnics of C, macropomum culttire. Normally, fish meal is the main protein source in experimental diets for C, macropomum. Fish meal combines the properties of good taste and high pro- tein quality, but it is also relatively expensive (Lovell 1989), Efforts to find cheaper dietary protein sources usually start with the substitution of fish meal by soya meal. Soya meal is generally considered the best vegetable protein source, because it has a relatively high protein content (betweeh 44 and 49%; Tacon 1987), an appropriate amino acid profile and high protein digestibility (Lovell 1991). In tilapia, channel catfish and rainbow trout, it was found that protein from solvent-extracted soya meal was equally or more digestible than fish meal protein (Lovell 1991), However, to judge protein qual- ity, one has not only to consider protein digestibility, but also the amino acid composition. The chemical score (CS) gives an indication of the resemblance between the indispensable amino acid (IAA) profile of dietary protein and that of body protein, and as such, can be used to judge the amino acid compo- sition of a diet (Hepher 1988), Based on CSs, and asstiming equal digestibility of soya and fish meal. Van der Meer & Verdegem (1995) calculated that 90% of the fish meal of a 'standard diet' could be substi- tuted by soya meal without affecting the growth of C. macropomum. Substitution of fish meal by soya meal as the main protein source has been investigated in carp (Viola, Mokady, Rappaport & Arieli 1982), channel catfish (Mohsen & Lovell 1990), African catfish (Machiels 1987), rainbow trout (Dabrowski, Poczyczynski, Kock &Berger 19 89) and shrimp (Lim&Dominy 1990).The general conclusion from the experiments mentioned above is that the effect of a partial substitution of fish meal by soya meal depends on the species studied; however, complete substitution restated in significantly © 1996 BlackweU Science Ltd. 531